1. Field of the Invention
The present invention relates to a method of compressing gas in a compressor station for a gas pipe line, particularly in permanent frost areas. The method includes compressing the gas delivered in the pipeline with an entry pressure in a compression procedure to a higher pressure, subsequently cooling the gas by a heat exchange and again feeding the gas for the further transportation to the pipeline with a lower exit temperature, particularly an exit temperature of at most 0.degree. C., and with an increased exit pressure as compared to the entry pressure.
The present invention also relates to an arrangement for carrying out the method.
2. Description of the Related Art
Natural gas is transported today in very large quantities frequently over distances of several thousand kilometers in large gas pipelines to the centers of consumption. For example, such long-distance gas pipelines may have a diameter of 56 inches and may be operated with gas pressures of 75 bar or even up to 100 bar, in order to achieve a transportation capacity which is as large as possible. Because of the unavoidable pressure loss along the gas pipelines, the compressor stations must be provided at certain intervals for increasing the gas pressure back to the nominal pressure. As a rule, the compressors used for this purpose, usually turbo compressors, are driven by gas turbines which use a portion of the transported natural gas as fuel. A very large portion of the known natural gas reserves are located in so-called permanent frost areas, i.e., in areas in which the ground thaws during the summer months only to a depth of about 80 to 100 cm and remains otherwise permanently frozen. The gas pipelines are usually placed at a depth in the ground where permanent frost prevails. Since the soil frequently becomes very soft in the thawed state, it must be ensured that the gas pipeline does not result in thawing of the ground because the pipeline would otherwise at least at certain locations sink lower and lead to mechanical stresses in the pipe wall which may lead to pipe ruptures. Heating of the soil is a possibility because the compression of the gas in the compressor inevitably also results in a temperature increase. Therefore, the gas compressed to nominal pressure is conventionally cooled before being returned into the pipeline, wherein a maximum temperature of approximately 0.degree. C. must be maintained. If possible, a temperature of - 5.degree. C. is desirable.
Because of the low outside temperatures substantially below 0.degree. C., the required cooling poses no problems during the winter months and can be easily carried out by gas/air coolers. However, during the transition periods and particularly in the summer months, during which maximum day temperatures of 15.degree. to 20.degree. C. are possible, the gas coolers are inevitably no longer sufficient. For this reason, special re-cooling plants with separate cooling cycle, i.e., refrigerating or cooling machines in which propane in particular is used as a cooling agent, are used in such compressor stations during the warm weather periods.
The use of re-cooling plants of the conventional type poses several problems. The re-cooling plants are very expensive and constitute a large portion of the total investment for a compressor station. In addition, there is the fact that the plant is completely unused during the major portion of a year, i.e., for eight months. In addition, there is a safety problem with respect to possible leakages because the propane as cooling agent is not only easily flammable, but is also heavier than air and, therefore, has a reduced volatility, so that the explosion risk is substantially increased.